Annealing single crystal chemical vapor deposition diamonds

ABSTRACT

A method to improve the optical clarity of CVD diamond where the CVD diamond is single crystal CVD diamond, by raising the CVD diamond to a set temperature of at least 1500 degrees C. and a pressure of at least 4.0 GPa outside of the diamond stable phase.

The present invention claims the benefit of priority from U.S.Provisional Application Ser. No. 60/486,435 filed Jul. 14, 2003, whichis hereby incorporated by reference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with U.S. government support under grant numberEAR-0135626 from the National Science Foundation. The U.S. governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to annealing diamond, and moreparticularly, to annealing single crystal CVD diamond.

2. Discussion Of The Related Art

Chemical vapor deposition growth of diamonds is achieved by impartingenergy into a gas-phase carbon containing precursor molecules. Forexample, microwave energy can be used to create plasma that depositscarbon so as form diamond upon a seed diamond. Up until recently, allCVD techniques for growing diamond resulted in polycrystalline diamondor very thin layers of single crystal diamond. The present inventors ofthis application have developed a microwave plasma CVD technique to growlarge single crystal CVD that is disclosed in copending patentapplication Ser. No. 10/288,499 filed on Nov. 6, 2002, which is herebyincorporated by reference.

The present inventors' microwave plasma CVD technique can grow a singlecrystal diamond on a seed diamond, such as a yellow type Ib HPHTsynthetic diamond, at rates of up to 150 micrometers an hour. The colorof the diamonds produced by the present inventors' microwave plasma CVDtechnique depends on the temperature at which the diamond is grown. Moreparticularly, when diamond is grown within a certain temperature range,which is dependent upon the mixture of gases in the plasma, a colorlessdiamond can be produced. However, diamonds produced at temperaturesoutside of the certain range are yellow or brown in color.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a annealing a singlecrystal CVD diamond that substantially obviates one or more problems dueto limitations and disadvantages of the related art.

An object of the present invention is to lighten or remove the colorfrom a single crystal CVD diamond.

Another object of the present invention is to eliminate defects from asingle crystal CVD diamond.

Another object of the present invention is to enhance the opticalcharacteristics of a single crystal CVD diamond.

Additional features and advantages of the invention will be set forth inpart in the description which follows, and in part will be apparent fromthe description, or may be learned from practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described, a method toimprove the optical clarity of single crystal CVD diamond that includesraising the CVD diamond to a set temperature of at least 1500 degrees C.at a pressure of at least 4.0 GPa.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention.

The present inventors have used their microwave plasma CVD technique togrow single crystal diamonds with thickness of greater than onemillimeter on a seed diamond, such as a type Ib {100} HPHT syntheticdiamond. In order to enhance the growth rate to about 50-150 μm/h andpromote smooth {100} face growth, the single-crystal CVD diamond wasgrown in an atmosphere of N₂/CH₄=0.2-5.0%, CH₄/H₂=12-20% with a 120-220torr total pressure, and at a temperature of 900-1500 degrees C. Ramanspectra show a small amount of hydrogenated amorphous carbon (a-C:H)⁴and nitrogen-containing a-C:H (N:a-C:H)⁴ that cause the single crystalCVD diamond to have a brown or yellow color when the diamond is grown at<950° C. or >1400° C. Further, photoluminescence (PL) spectra indicatenitrogen-vacancy (N-V) impurities.

The CVD single crystal diamond grows faster at the higher temperaturesof 1400-1500 degrees but the resulting diamond has a brown color.Regardless of the color, the single crystal CVD diamonds resulting fromthe present inventors' microwave plasma CVD technique can still haveimpurities. Of course, a lighter colored diamond, a more translucentdiamond or a diamond having no color is desired.

The lightening of the brown color in brown natural diamonds, as well asa decrease in impurities by high pressure high temperature annealing hasbeen reported by I. M. Reinitz et al. Gems & Gemology 36, 128-137(2000). However, the present inventors have discovered that HPHTannealing of a single crystal yellow or brown CVD diamond at atemperature of 1800-2900° C. and at a pressure of 5-7 GPa for about 1-60minutes using a reaction vessel in a conventional high pressure hightemperature apparatus so as to transform some single crystal brown CVDdiamonds into transparent colorless single crystal diamond. Moreparticularly, a single crystal yellow or light brown CVD diamond grownwith high growth rate at a temperature of about 1400-1460 degrees C. inan atmosphere containing 4-5% N₂/CH₄ ratio can be annealed to become acolorless single crystal diamond. Further, Raman and PL spectra of suchan annealed CVD diamond demonstrate the disappearance of hydrogenatedamorphous carbon and a significant decrease in N-V impurities in such acolorless single crystal diamond. These changes appear to be similar tothe report by I. M. Reinitz et al. of transparency enhancement producedby HPHT annealing of brown natural diamond.

The reaction vessel can be a cell, such as that described in U.S. Pat.Nos. 3,745,623 or 3,913,280, which are hereby incorporated by reference.The entire cell is subjected to pressures in excess of 4.0 GPa, such as5-7 GPa, and heated to temperature in excess of 1500 degrees C., such as1800-2900 degrees C., such that the pressure/temperature conditions inthe cell or reaction vessel are within the graphite stable phase or justwithin the diamond stable phase for a time period that can be as shortas 1 minute or as long as an hour. The cell is allowed to cool prior tothe pressure being released so that the single crystal CVD diamond doesnot become graphite.

EXAMPLE #1

A single crystal CVD diamond was grown with a N₂/CH₄ ratio of 5% at atemperature of approximately 1500 degrees C. on a yellow type Ib HPHTsynthetic diamond at the Carnegie Institution of Washington. Thedimension of the single crystal diamond CVD diamond was one centimetersquare and a little larger than one millimeter in thickness. The colorof the single crystal diamond CVD diamond was brown. The brown singlecrystal CVD diamond on the type Ib HPHT synthetic seed diamond was thenplaced as a sample in the reaction vessel.

The reaction vessel was placed in a conventional HPHT apparatus. First,the pressure was increased to a pressure of 5.0 GPa, and then thetemperature was rapidly brought up to 2200 degrees C. The sample wasmaintained at these annealing conditions for five minutes, and then thetemperature was decreased over a period of about one minute to roomtemperature before the pressure was released.

The sample was removed from the reaction vessel and examined under anoptical microscope. The brown single crystal CVD diamond had turned to alight green color and remained firmly bonded to the yellow type Ib HPHTsynthetic diamond. The yellow color of the synthetic type Ib diamondbecame a lighter yellow or a more translucent yellow.

EXAMPLE #2

Same as example #1 above, except the annealing conditions weremaintained for 1 hour. The dark brown single crystal CVD diamond turnedto a light green color, which was more translucent than the light greencolor resulting in example #1, and remained firmly bonded to the yellowtype Ib HPHT synthetic diamond. The yellow color of the type Ib HPHTsynthetic diamond became a lighter yellow or a more translucent yellow.

EXAMPLE #3

A single crystal CVD diamond was grown with a N₂/CH₄ ratio of 5% at atemperature of approximately 1450 degrees C. on a yellow synthetic typeIb diamond at the Carnegie Institution of Washington. The dimension ofthe single crystal diamond CVD diamond was one centimeter square and alittle larger than one millimeter in thick. The color of the singlecrystal diamond CVD diamond was a light brown or yellow. In other words,a light brown that was not as dark as the brown of the single crystalCVD diamond in example #1 above. The yellow or light brown singlecrystal CVD diamond on type Ib HPHT synthetic seed diamond was thenplaced as a sample in a reaction vessel.

The reaction vessel was placed in a conventional HPHT apparatus. Thepressure was increased to about to a pressure of 5.0 GPa, and then thetemperature was rapidly brought up to about 2000 degrees C. The samplewas maintained at these annealing conditions for five minutes, and thenthe temperature was decreased over a period of about one minute to roomtemperature before the pressure was released.

The sample was removed from the reaction vessel and examined under anoptical microscope. The light brown or yellow single crystal CVD diamondhad become colorless and remained firmly bonded to the yellow type IbHPHT synthetic diamond. The yellow color of the type Ib HPHT syntheticdiamond also became a lighter yellow or a more translucent yellow.

EXAMPLE #4

Same as example #1 except a colorless microwave plasma single-crystalCVD-grown diamond in an atmosphere of N₂/CH₄=5% at a temperature of 1200degree C. was annealed. After annealing, the microwave plasmasingle-crystal CVD-grown diamond was blue. This blue microwave plasmasingle-crystal CVD-grown diamond had a very high toughness of >20 MPamn^(1/2). The hardness was about 140 GPa.

EXAMPLE #5

Same as example #1 except a colorless microwave plasma single-crystalCVD-grown diamond in an atmosphere of N₂/CH₄=0.5% at a temperature of˜1200 degree C. was annealed. The microwave plasma single-crystalCVD-grown diamond was still colorless. This colorless microwave plasmasingle-crystal CVD-grown diamond had a hardness of 160 GPa and toughnessof ˜10 MPa m^(1/2).

As the present invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims, and therefore allchanges and modifications that fall within the metes and bounds of theclaims, or equivalence of such metes and bounds are therefore intendedto be embraced by the appended claims.

1. A method to improve the optical clarity of CVD diamond where the CVDdiamond is single crystal CVD diamond, by raising the CVD diamond to aset temperature of at least 1500 degrees C. and a pressure of at least4.0 GPa outside of the diamond stable phase.
 2. The method of claim 1wherein, the CVD diamond is a single crystal coating upon anothermaterial.
 3. The method of claim 1, wherein the step of raising thetemperature of the single crystal CVD diamond further comprises the stepof: raising the single crystal CVD diamond to a set temperature of about1800 degrees C. to about 2900 degrees C.
 4. The method of claim 1,wherein the step of raising the temperature of the single crystal CVDdiamond further comprises the step of: maintaining the temperature ofthe single crystal CVD diamond at the set temperature for less thanabout one minute.
 5. The method of claim 1, wherein the step of raisingthe temperature of the single crystal CVD diamond further comprises thestep of: raising the temperature of the single crystal CVD diamond to atleast 1500 degrees C. over a time period of about one minute to fiveminutes.
 6. The method of claim 1, wherein the step of raising thetemperature of the single crystal CVD diamond comprises of the step of:raising the temperature of the CVD diamond to about 2200 degrees C. at apressure of about 5.0 GPa.
 7. The method of claim 1, further comprisingthe step of: after reaching the set temperature, decreasing thetemperature of the CVD to ambient temperature while maintaining thepressure on the single crystal CVD diamond.
 8. The method of claim 1,wherein the single crystal CVD diamond initially has a brown color andbecomes colorless.
 9. The method of claim 1, further comprising the stepof: growing the single crystal brown diamond at a temperature of about1400-1460 degrees C. in an atmosphere containing 4-5% N₂/CH₄.